To remove the paint, dirt or other surface coating from a substrate such as a surface to be painted or cleaned, a blasting system is desirable and effective. There are a variety of blasting processes for these purposes, including, but not limited to, water blasting, dry abrasive blasting, wet abrasive blasting, and wet cutting systems, for example. In certain applications, blasting processes are able to efficiently cut, clean or remove a coating from a substrate.
Though generally safe, blasting systems include several safety precautions. Blasting systems are designed to spray high velocity streams of compressed air, fluids, and/or abrasive particles on a surface. A misdirected blast spray stream may cause harm to an operator.
Further, all types of abrasive and water blast systems may comprise an automatic control circuit to shut off the flow to the spray nozzle if the nozzle is dropped or a handle is otherwise released. These handles are generally called “dead man's switches.” For abrasive or water blasting systems, a dead man's switch is typically a spring-loaded handle that must be biased against the spring and sufficiently depressed to start and maintain the blasting operation. If the handle is released, the spring biases the handle to its original position and the flow of abrasive to the blast nozzle is stopped. As such, a dead man's switch is designed to automatically shut down the blasting operation in case the operator becomes incapacitated or otherwise loses control of the hose or nozzle, such as by injury, loss of consciousness, death, or other reason. The blasting processes may also be deliberately shut down using the dead man's switch.
Conventionally, dead man's switches may be either pneumatically controlled or electrically controlled. More often for blasting systems, abrasive blasters are controlled pneumatically with a pneumatic dead man's switch. Since abrasive blasters are typically powered by compressed air, it is logical to utilize some of the compressed air to operate the control circuits, regulate pressures, and to turn on and off relays and diaphragm valves, etc. Traditionally, the pneumatic dead man's switch acts to turn on or off an air relay valve that in turn controls diaphragm valves and air regulators that control (turn on and off) the flow of the blasting media.
Conventionally, most blast systems comprise a pneumatically operated dead man system. However, currently, the rules of the Occupational Safety and Health Administration (OSHA) may require that a dead man circuit be electrically operated on abrasive blast systems if the blast hose exceeds 150 feet in length. Over such distances, electrically operated dead man's switches can shut down the blast stream faster than pneumatically operated systems. A longer pneumatic hose is filled with more air and it takes a longer time for the pressure within the hose to drop sufficiently to switch the relay valve. This is due to a “balloon effect” developed in the long runs of pneumatic lines. The balloon effect results in a slower release of pressure from the control line 14b and therefore, the shut-off valves remain open for a time period after the handle of the dead man's switch is released. OSHA rules state that this balloon effect results in too long of a time delay between the release of the handle and the closing of the shut-off valves if the air hoses are longer than 150 feet.
In most cases for blasting systems, it is more convenient and less expensive to pneumatically control the dead man circuit. However, there are various applications for blasting systems and, consequently, an abrasive blasting system comprising a pneumatically operated dead man's switch may need to be converted to an electrically operated dead man's switch. Conventionally, the conversion involved the expensive replacement of the air relay valve, the replacement of the blast hose dead man airline with electrical cords, the replacement of the dead man's switch from pneumatic to electric switch, as well as re-plumbing internal control lines or hoses. A complicated portion of this conversion is the replacement of the air relay or components, re-routing the tubing and wiring of the air relay valve, as well as making modifications to the internal control tubing circuits.
There is a need for a blast system with a simple dead man control circuit that may be easily converted from a pneumatically controlled system to an electrically controlled system. In the past, switching from pneumatic to electric control of a dead man safety system has been overly complicated and required extensive training before a technician became fully proficient in the conversion.